Plants host a large diversity of microorganisms, which includes fungi, bacteria and archaea. Among these, fungi are highly diverse, and known to play a vital role in plant health and in regulation of the essential ecosystem functions. Nevertheless, we still lack a comprehensive understanding of the forces structuring plant-associated fungal communities in space and time. 

The main aim of this thesis was to decipher the drivers of the spatial patterns and temporal dynamics of fungal communities on plants. To this aim, I focused on Quercus robur and its associated fungi. Using a combination of observational and experimental studies, I assessed i) the distribution and drivers of the above- and belowground fungal communities at the landscape scale; ii) the role of climatic and trophic factors in defining the niches of cryptic species within a pathogen complex on oak and iii) the relative importance of warming, plant genotype and their interaction in shaping oak phenology and the seasonal dynamics of the associated fungal and insect communities.

I found that aboveground fungal communities were highly variable among leaves within a single tree, and that belowground fungal communities had a stronger spatial structure than aboveground fungi at the landscape scale. Yet, climate, tree phenology or the distribution of the host tree did not explain spatial patterns in the above- and belowground communities. When focusing on three cryptic powdery mildew species within a pathogen complex on oak, I demonstrated that the climatic dimension is more important than the species interaction dimension for niche differentiation of these cryptic pathogens. A field heating experiment showed strong seasonal change in the structure of the foliar fungal community, with experimental warming playing an important role in driving this change. This experiment also revealed that warming and plant genotype jointly shape plant phenology, disease levels and insect abundance across the growing season.

In conclusion, my findings suggest that abiotic forces can override biotic forces in structuring spatial patterns and temporal dynamics of fungal communities associated with plants. The particularly strong impact of warmer temperatures on foliar fungi in some of my studies indicates that climate warming has the potential to structure foliar fungal communities, with important implications for plant health, interactions between plants and other organisms and ecosystem functions.